Merriam-Webster's Collegiate® Dictionary, 11th Edition
TONIC
I. adjective
Etymology: Greek tonikos, from tonos tension, tone
Date: 1649
1. characterized by tonus , producing or adapted to produce healthy muscular condition and reaction of organs (as muscles),
2. increasing or restoring physical or mental tone ; refreshing , yielding a tonic substance, relating to or based on the first tone of a scale , bearing a principal stress or accent, of or relating to speech tones or to languages using them to distinguish words otherwise identical, tonically adverb
II. noun
Date: 1797
the first tone of a major or minor scale ; keynote ,
2. an agent (as a drug) that increases body tone, one that invigorates, restores, refreshes, or stimulates , a liquid preparation for the scalp or hair, a carbonated flavored beverage, tonic water , a voiced sound